Just got back.
First off, the 3d effect is pretty subtle for most of the movie but really gives you some good depth perception when you have things staggered at different distances. This is a very good thing, and helps to draw you into the movie so you don't even realize you're watching a movie at times. It definitely helps you feel like you're "there". Now for some of the issues I had with the 3d. One issue was that because I didn't go to Imax 3d, the center of the screen was the only place I really could get "into" the 3d. Around the boarders I couldn't easily because the flat edges in the theater gave my eyes perspective which killed the 3d effect. The biggest issue with the 3d were two fold (although I believe symptoms of the same core issue). There were pan scenes where it would stutter, which Cameron has already addressed. The bigger problem with this was when there were either like butterflies or something that was at the edge coming into the scene, or the camera moving back and something just entering the cameras FoV. It would kill the 3d effect and looked bad IMHO. Overall though the 3D I'd give a 9/10.
Onto the actual movie. The plot was very predictable, and a story you've seen before. That said it was still good. It was very well written, and a very satisfying movie. You shouldn't go into it expecting to see some new amazingly new and creative story, because you'll be let down. Go in with an open mind, and look at how well that same old story is told this time. I enjoyed the movie, and think people should go see it for the well written story that it is. Oh, and FWIW people went into Titanic knowing what was going to happen and the basic plot. It still is the all time highest grossing movie as of now. I'd give the movie itself a 7.5/10.
I'm going to go see it again I think in Imax 3d. It's worth it if you have the right frame of mind (i.e. not expecting some new story with major plot twists and such). Overall I'd give it a solid 8/10, and everybody should see it in theaters.
Also, I heard a rumor that this will NOT be released on DVD/Blu-ray since you can't get the 3d at home as of now. Anybody know if that's true?